Diffractive stack pickup head for optical disk drives and method to fabricate the pickup head

ABSTRACT

The invention provides a pickup head used in an optical data storage system. The pickup head contains a diffractive optical element (DOE) stack on a semiconductor substrate. The DOE stack includes multiple diffractive lenses for providing several diffractive surfaces and a middle layer serving as a beamsplitter and servo-generating element for a light reflected from an optical storage medium. The middle layer is sandwiched by the diffractive lenses that are located on both outer parts of the DOE stack. The semiconductor substrate includes at least a laser source and several photodetectors. The middle layer serving its function preferably includes a polarization-selective DOE and a quarter-wave retarder oriented to rotate a polarization state of the laser source, so that only the light reflected from the optical storage medium is diffracted by the polarization-selective DOE.

CROSS REFERENCE RELATED TO APPLICATION

This application is a divisional application of, and claims the prioritybenefit of, U.S. application Ser. No. 09/434,864 filed on Nov. 4, 1999,now U.S. Pat. No. 6,351,443, issued on Feb. 26, 2002.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to an optical electronic technology. Moreparticularly, the present invention relates to a diffractive stackpickup head for an optical disk drive and a method to fabricate thediffractive stack pickup head.

2. Description of Related Art

Pickup heads are an opto-electronic means by which a signal is writtento or read from an optical disk. FIG. 1 is a system drawing,schematically illustrating a conventional optical disk pickup system. InFIG. 1, the generic optical disk pickup system includes a laser diodelight source 20, a beamsplitter 22, and at least one lens 24 forprecisely focusing the light from the laser diode 20 onto an informationbearing surface of the optical disk 26. The system further includes amulti-element photodetector 28 for receiving light that is reflectedfrom the disk 26, and a servo-generating element 30, such as thecylindrical lens, which is used to alter the reflected beam from thedisk 26 into some form whereby the state of focus and tracking can beeasily extracted from the signals generated by the light incident on themulti-element photodetector 28. This conventional system can be referredto A. B. Marchant, Optical recording, A Technical Overview,Addison-Wesley Publishing, Menlo Park Calif., 1990, p.197. The generalmounting arrangement for these optical elements into a pickup head unitis shown in FIG. 2. In FIG. 2, each component is mounted separately, oneat a time, into a pickup head housing 32. The size of the housing 32 isdirectly related to the image distance of the focusing lens(es) 24,i.e., the distance from the laser diode 20 to the focused image of thelaser diode 20 on the disk 26 of FIG. 1. With current technology ofrefractive molded aspheric lenses, this distance is typically in therange of 25-40 mm, and the size of the housing is roughly of the sameorder. In some designs, the servo-generating element can be an integralpart of the beamsplitter 22 and sometimes is implemented using adiffractive or holographic optical element (DOE). FIG. 3 is a pickupsystem drawing, schematically illustrating an alternate generic pickupdesign where the beamsplitter and servo-generating elements are combinedinto a single DOE and the complete optical system excluding theobjective lens is packaged in a small module. In FIG. 3, the design ofthe pickup system 38 has the advantage that most of the opto-electroniccomponents 40, except the focusing lens/actuator 42, are assembled in aminiature module. The components 40 are also assembled one at a time toform the miniature module. To complete the pickup system, the components40 are mounted in a pickup housing similar to the one shown in FIG. 2.However, the general size of the pickup system 38 is still determined bythe imaging distance of the focusing lens. In other words, theholographic module approach has certain advantages but it does notsignificantly change the size or weight of the complete pickup system.

SUMMARY OF THE INVENTION

The invention provides an optical disk pickup head with the currentstate-of-the-art technology having an objective that the inventionallows many pickups to be made simultaneously in parallel, therebysignificantly reducing manufacturing cost.

The invention also includes another objective that the entire pickup,including the focusing lens, can be made in a very small dimension, suchas a few millimeters on a side. This allows multiple pickups to beutilized in a single drive.

As embodied and broadly described herein, the invention provides apickup head used in an optical data storage system. The pickup headincludes a diffractive optical element (DOE) stack on a semiconductorsubstrate. The DOE stack includes multiple diffractive lenses forproviding several diffractive surfaces and a middle layer serving as abeamsplitter and servo-generating element for a light reflected from anoptical storage medium. The middle layer is sandwiched by thediffractive lenses that are located on both outer parts of the DOEstack. The semiconductor substrate includes at least a laser source andseveral photodetectors.

The middle layer's function preferably includes a polarization-selectiveDOE and a quarter-wave retarder oriented to rotate the polarizationstate of the laser source, so that only the light reflected from theoptical storage medium is diffracted by the polarization-selective DOE.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings,

FIG. 1 is a system drawing, schematically illustrating a conventionaloptical disk pickup system;

FIG. 2 is a dissected perspective view, schematically illustrating aconventional mounting structure of optoelectronic components into apickup head unit;

FIG. 3 is a pickup system drawing, schematically illustrating analternate generic pickup design where the beamsplitter andservo-generating elements are combined into a single DOE and thecomplete optical system excluding the objective lens is packaged in asmall module;

FIG. 4 is a cross-sectional view, schematically illustrating a pickuphead including the DOE stack formed on a semiconductor substrate,according to the preferred embodiment of the present invention;

FIG. 5 is a drawing, schematically illustrating a concept of afabrication method for the DOE stack according to the preferredembodiment of the invention, in which a 2-dimensional array of elementsis first fabricated on each wafer layer, and then the wafer layers arealigned and bonded together where the wafers are cut into strips of1-dimensional arrays of DOE stack elements or into individual DOE stackelements;

FIG. 6 is a top view, schematically illustrating the structure of thepickup head on a the semiconductor substrate, according to the preferredembodiment of the invention;

FIG. 7 is a drawing, schematically illustrating a method for mountingthe strips of 1-dimensional arrays of DOE stack elements onto the arrayof laser diode/multi-element photodetector systems that has beenfabricated on the semiconductor substrate wafer, according to thepreferred embodiment of the invention;

FIG. 8 is a drawing, schematically illustrating a pickup system usingthe DOE stack according to a first preferred embodiment of theinvention, in which light path through the DOE stack is folded byseveral reflective DOE surfaces and the DOE stack contains bothreflective and transmissive DOE surfaces;

FIG. 9 is a drawing, schematically illustrating a structure for thebeamsplitter/servo-generating DOE and the multi-element photodetectors,according to the first preferred embodiment of the invention;

FIG. 10 is a drawing, schematically illustrating the DOE for thebeamsplitter and the servo-signal generating element, according to thefirst preferred embodiment of the invention; and

FIG. 11 is a drawing, schematically illustrating a DOE stack, accordingto a second preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention utilizes a stack of diffractive optical elements (DOE) incontact with a semiconductor substrate to build a pickup head. The basicstructure is shown in FIG. 4. FIG. 4 is a cross-sectional view,schematically illustrating a pickup head including the DOE stack formedon a semiconductor substrate, according to the preferred embodiment ofthe present invention. In FIG. 4, a DOE stack 44 is located on asemiconductor substrate 46. The DOE stack 44, for example, includes atleast one diffractive lens surface 44 a, such as two, on the top of theDOE stack 44, and includes at least one diffractive lens surface 44 b onthe bottom of the DOE stack 44, where the at least one diffractive lenssurface 44 b is preferably shown in two. The diffractive lenses 44 a, 44b provides several diffractive lens surfaces. The DOE stack 44 alsoincludes a quarter-wavelength retarder 44 c and a polarization-selectiveDOE 44 d performing a function of a beamsplitter & servo-signalgeneration. The quarter-wavelength retarder 44 c and apolarization-selective DOE 44 d are sandwiched by the diffractive lenses44 a, 44 b, and the quarter-wavelength retarder 44 c is preferablylocated on the polarization-selective DOE 44 d. The DOE stack 44 canalso perform a focusing function. Moreover, the semiconductor substrate46 usually includes at least a laser diode and several photodetectors,for example, like those shown in FIG. 2.

During manufacturing, each layer in the DOE stack 44 is treated aswafer-like, and is patterned by standard semiconductor processingtechnologies, such as processes of photolithography and etching. Thepattern of each layer includes, for example, a regular array of aparticular shape that is required for each individual layer according tothe pattern design. In other words, many identical copies of thediffractive surface pattern required by the design for a given layer ofthe DOE stack are fabricated simultaneously on the wafer. FIG. 5 is adrawing, schematically illustrating a concept of a fabrication methodfor the DOE stack according to the preferred embodiment of theinvention, in which a 2-dimensional array of elements is firstfabricated on each wafer layer, and then the wafer layers are alignedand bonded together where the wafers are cut into strips of1-dimensional arrays of DOE stack elements or into individual DOE stackelements. In FIG. 5, as each layer of the DOE stack 44 is formed withthe desired pattern, each layer is stacked up to form a preliminary DOEstack 44′ with a two-dimensional pattern. The preliminary DOE stack 44′is then, for example, cut into several strips. Each strip is a set ofthe DOE stack 44 of FIG. 4. The layers on the outsides of the DOE stack44 are the diffractive lens surfaces 44 a, 44 b of FIG. 4 and arecomposed of, for example, glass, plastic, or other similar material.

The diffractive lens surfaces 44 a, 44 b can diffract light for any andall polarization states. The focusing power, which images the laserdiode onto the disk information surface with a high numerical aperture(NA) convergence of the light on the disk side, is distributed betweenthree or four diffractive lens surfaces 44 a, 44 b of the DOE stack. Intheory, the focusing power could be totally realized with only one ortwo DOE's. The requirement of high-NA, such as NA=0.6 for a DVD pickuphead, leads to a fine pitch for the grooves of the DOE, which makes themdifficult or impossible to manufacture and adversely affects thediffraction efficiency of the DOE stack 44. By distributing the focusingpower between three or four surfaces uniformly, high diffractionefficiency can be achieved and the minimum groove spacing is increasedmaking fabrication feasible. Furthermore, it is well known that DOE'sare highly dispersive. By using multiple diffractive surfaces thattogether form the focusing lens, one can design these surfaces with abalance of positive and negative powers to achieve the desired amount ofachromaticity. This cannot be achieved by only using a singlediffractive surface.

The sandwiched layers 44 c and 44 d of the DOE stack 44 of FIG. 4 alsoinclude another DOE surface. This surface implements the functions ofbeamsplitter and servo-generating element. The DOE pattern and themulti-element photodetector pattern, as mentioned in FIG. 5, aredesigned such that one can determine the information recorded on thedisk as well as the state of focus and tracking of the light spot on thedisk from the electrical signals. The electric signals are generated bythe light incident on the photodectors. The servo-generating andbeamsplitting function can be implemented using either one or twolayers. When implemented with just one layer, this single layer replacesthe two layers 44 c and 44 d shown in FIG. 4. This single layer is madefrom an isotropic material and diffracts light of any polarizationstate. The forward-going light (i.e. towards the disk) that isdiffracted by this layer is not used. The return-path light, which isreturning due to reflection from the disk, is diffracted onto thephotodetectors. Alternatively, the servo-generating/beamsplitterfunctions may be implemented using two layers 44 c and 44 d as shown inFIG. 4. These two layers are either made from birefringent materials orby other means fabricated such that their influence on an optical beampassing through them depends on the polarization of the incident beam.The layer nearest the laser diode, 44 d, has a polarization-selectiveDOE fabricated on one surface. The laser emits linearly polarized light.As it propagates from the laser towards the disk, thepolarization-selective DOE is designed, and its birefringence axis isoriented, such that no diffraction occurs. The otherpolarization-dependent layer, 44 c, i.e., the one nearer the disk, isfabricated as a quarter-wave retarder and is oriented such that its axisis at 45 degrees to the direction of polarization emitted by the laser.Passing through this quarter-wave retarder layer 44 c twice, once as itpropagates from the laser to the disk and a second time as the lightreflected from the disk passes back through the pickup head, thepolarization direction of the light is rotated by 90 degrees. As aresult, when the light reflected from the disk reaches thepolarization-selective DOE surface 44 d, it is now of the correctpolarization orientation to be diffracted with high efficiency by thissurface onto the photodetectors.

The laser diode and the multi-element photodetectors are fabricated ormounted on the semiconductor substrate 46. The basic layout of thecomponents on the substrate 46 is shown in FIG. 4 and further shown inFIG. 6. FIG. 6 is a top view, schematically illustrating the structureof the pickup head on the semiconductor substrate, according to thepreferred embodiment of the invention. The structure shown in FIG. 6 isone unit of a laser/photodetector system element 60. In FIG. 6, thesemiconductor substrate 46 includes an indentation or well 50 where thelaser diode 52 is located. The laser diode 52 has a reflecting surface(mirror), also called mirror/beamshaper 53, in front of the laseremission facet to turn the laser beam by 90 degrees so that the laserbeam exits perpendicular to the plane of the semiconductor substrate 46.This reflecting surface may also have curvature or diffractive power soas to perform beamshaping on the laser beam. In the production process,a semiconductor wafer 46 is patterned with an array of the laser andmulti-element photodetector systems 60 as described above. Each step ofthe fabrication process is performed on all array elements in parallel,such as a typical semiconductor fabrication. In other words, severalpickup elements are fabricated on each semiconductor wafer at the sametime. Metal traces 54 are also formed to allow several bonding pads 56formed on the substrate 46 to be electrically coupled to thephotodetectors 48. It must be noted that the bonding pads 56 should notbe covered when the DOE stack 44 is mounted on the semiconductorsubstrate 46.

As the DOE stack 44 is to be assembled on the semiconductor substrate46, the optical components of the DOE stack 44 must be properly alignedand bonded to the optoelectronic laser diode and the photodetectorsystem on the semiconductor substrate 46. There are various ways toassemble the DOE stack 44. The semiconductor substrate 46 and the DOEstack can be each diced into individual pickup elements and bonded oneby one. Or the DOE stack 44 can be diced into individual elements, andthen each is mounted on the semiconductor substrate 44 at the desiredplace. FIG. 7 is a drawing, schematically illustrating a method formounting the strips of 1-dimensional arrays of DOE stack elements ontothe array of laser diode/multi-element photodetector systems that hasbeen fabricated on the semiconductor substrate wafer, according to thepreferred embodiment of the invention. In FIG. 7, several pickupelements are fabricated in parallel. The DOE stack 44 is a strip cutfrom the preliminary DOE stack 44′ of FIG. 5. Each DOE stack 44 includesan one-dimensional array of DOE stack elements, which have the samespacing as the laser/photodetector system elements 60 on thesemi-conductor substrate 46. The one-dimensional array strips of DOEstack 44 can be aligned row by row with respect to rows of thelaser/photodetector system elements 60, each of which has an opening toexpose the bonding pads for leading electric signals between the DOEstack array strips 44. After the mounting the DOE stack 44 on thesemiconductor substrate 46, the substrate 46 is diced into individualpickups.

The pickup fabricated by the above method of the invention can havevarious applications in the optical pickup head. Several embodiments forthe pickup system using the DOE stack are, for example, shown in thefollowing.

Embodiment 1

FIG. 8 is a drawing, schematically illustrating a pickup system using aDOE stack according to a first preferred embodiment of the invention, inwhich light path through the DOE stack is folded by several reflectiveDOE surfaces and the DOE stack contains both reflective and transmissiveDOE surfaces. In FIG. 8, a distinctive part of this embodiment is thatreflective DOE surfaces 1 and 2 are used as two of the DOE lenssurfaces. The need for a high NA on the disk side of the lens and aworking distance on the order of a millimeter essentially determines thebeam size at the lens. The comparatively low NA of the beam emitted bythe laser diode 52 requires a certain distance to expand to this size,longer than the lens to the disk image distance by a factor of the ratioof lens NA to the beam NA. In this embodiment 1, a technique similar toa reflecting telescope, which folds the optical distance into a smallspace, is used. Light enters the diffractive stack through at least atransparent opening at the reflective DOE surface 2. It is incident onreflective DOE surface 1 that is designed to reflect the beam and toexpand its divergence. In this design, we use a diffractive surface todiverge the beam, which allows the surface to remain flat and,therefore, stackable. The beam is diverged at DOE surface 1, therebyincreasing the beam NA so that it takes less optical path length toreach the desired beam cross-sectional diameter. This also, in turn,allows the pickup to be fabricated in a smaller dimension. The DOEsurface 2 again reflects the beam back in the direction of the disk 26.Depending on the particular design, it may have converging, diverging,or have no focusing power. This degree of freedom in the design can alsobe used to achromatize the diffractive stack lens. In summary, byfolding the beam with reflection surfaces, light passes through thefirst layer of the diffractive stack, for example, three times. Thisresults in that the needed optical path length is folded into a smallerphysical space. By increasing the beam NA at DOE surface I and perhapssurface 2, the required optical path length is also reduced.

As the light beam again passes by DOE surface 1, the reflective DOEblocks a portion of the light beam, but most portion of the light beampasses through the transparent region outside of the DOE reflectiveregion. The beam polarization and the orientation of thepolarization-selective DOE 44 d are designed so that the forward-goinglight, which propagates from the laser diode 52 to the disk 26, is notdiffracted but rather passes directly through the polarization-selectiveDOE 44 d. After transiting the quarter-wave retarder 44 c, the linearlypolarized light becomes a circularly polarized light. The diffractiveDOE surface 3 and 4 both focus the light so that the desired high-NAconverging beam is incident on the disk 26. The diffractive powers ofthe DOE surfaces 1-4 are designed together so as to minimize thephysical size of the pickup and to share the required total focusingpower. In this manner, the pickup can achieve a high diffractionefficiency, a reasonable minimum groove spacing on the individual DOEsurfaces, and the necessary degree of achromaticity.

After reflection from the disk 26, the light beam again travels throughthe DOE surfaces 4 and 3, and the quarter-wave retarder 44 c. The netresult of traversing the properly-oriented quarter-wave retarder 44 ctwo times, is that the light emerges linearly polarized withpolarization direction rotated by 90 degrees relative to the lightemitted by laser diode 52. This is the polarization direction for whichdiffraction by the polarization-selective DOE is maximized. Thepolarization-selective DOE 44 d, serving the role of a beamsplitter,diffracts the light beam towards the multi-element photodetectors 48(see FIG. 6) on the semiconductor substrate 46, accessible throughtransparent openings at the DOE surface 2. The polarization-selectiveDOE 44 d also play the role of a servo-generating element, which is usedin conjunction with the multi-element photodetectors 48 to produce anelectronic signal that is used to determine whether or not the light isproperly focused on the disk 26 at its surface. The signal is alsocalled the focus error signal (FES). A tracking error signal (TES) isalso produced to determine whether or not the focused spot is properlyfollowing the track of information on the disk 26. In general, thepolarization-selective DOE 44 d and the photodetectors 48 can bedesigned to implement any of a number of well-known servo-generatingmethods, including but not limited to the astigmatic method, theknife-edge method, or the spot-size detection method for generating FESsignals. Moreover, the TES signals can be generated by, for example, apushpull (PP) and differential phase detection (DPD) method. Theradio-frequency (RF) signal containing the information stored on thedisk 26 is the sum of the electrical signals from all of themulti-element photodetectors 48.

A preferred method to form a compatible servo-generating element isdescribed in the following. FIG. 9 is a drawing, schematicallyillustrating a structure between the beamsplitter/servo-generating DOEand the multi-element photodetectors, according to the first preferredembodiment of the invention. In FIG. 9, the optical beam can besubstantially centered on this polarization-selective DOE 44 d. It isdivided into four regions, each of which directs part of the light beamfrom the disk onto a different photodetector element 48. FIG. 10 is adrawing, schematically illustrating the DOE for the beamsplitter and theservo-signal generating element, according to the first preferredembodiment of the invention. The light may either go directly from thepolarization-selective DOE 44 d to the photodetectors 48 as shown in thelower drawing, or it may be reflected by the reflective DOE surfacesbefore it reaches the photodetectors as shown in the upper drawing.

Both FIG. 9 and FIG. 10 schematically illustrate one preferred designfor the beamsplitter/servo-generating DOE 44 d and the multi-elementphotodetectors 48 that is compatible with the stack pickup designs ofthe invention. The X-axis corresponds to the tangential disk directionand the Y-axis to the radial disk direction. In FIG. 9, the parts of thelight beam reflected from the disk, falling on the DOE AB and the DOECD, are diffracted onto the multi-element photodetectors AB and CDrespectively. The DOE AB and the DOE CD in conjunction with themulti-element photodetectors AB and CD implement a method of focus errordetection. The focus error signal is given by:

FES=(SA−SB)−(SC−SD),

where SA, SB, SC, and SD indicate the electrical signals generated bylight falling on the photodetectors A, B, C, and D respectively. Theaxis of the central strips of the multi-element photodetectors 48 AB andCD, such as the strips B and D, are at +/−45 degrees relative to theY-axis. In this manner, the optical spots diffracted by DOE AB and DOECD remain centered on the central strips of detectors AB and CDindependent of slight wavelength shifts of the laser light thatinvariably occur with aging or temperature change. As a result, theproper FES is maintained despite shifts in wavelength. The parts of thebeam reflected from the disk that fall on DOE E and DOE F are diffractedonto photodetector elements E and F respectively. They are used toprovide DPD and pushpull (PP) tracking error signals as given by thefollowing expressions:

TES _(DPD)=phase(SA+SB+SE)−phase(SC+SD+SF),

TES _(pp)=(SA+SB+SC+SD)−(SE+SF),

where SE and SF indicate the electrical signals generated by lightfalling on photodetectors E and F respectively. The RF signal containingthe information stored on the disk is the sum of the electrical signalsfrom all of the multi-element photodetectors 48. In this embodiment 1,the photodetectors are on the opposite side of the optical axis, (thatis, the axis running from the laser diode to the focused spot on thedisk), from the quadrant elements of DOE AB and DOE CD. The method ofthe invention makes use of the reflective and diffractive functions ofDOE surfaces 1 and 2 in the optical path of the light diffracted by DOEAB and DOE CD of the polarization-selective DOE 44 d to bring the lightacross the optical axis and onto the photodetectors AB and CD. This isschematically shown in FIG. 10. In FIG. 10, the upper drawing shows thepath of the light diffracted by DOE AB or DOE CD and the lower drawingshows the path of the light diffracted by DOE E or DOE F.

Embodiment 2

FIG. 11 is a drawing, schematically illustrating a DOE stack, accordingto a second preferred embodiment of the invention. In FIG. 11, lighttravels through the DOE stack 44 in one direction from the laser diodeto the disk, and in the reverse direction from the disk to thephotodetectors. The embodiment 2 of FIG. 11 is basically similar to theembodiment 1 of FIG. 8, but the DOE stack 44 contains only transmissivesurfaces and no reflective surfaces. All DOE surfaces 1′, 2′, and 3′ arenow transmissive. Taking the optical axis from the laser to the focusedspot on the disk to be the Z-direction, the light travels completely inthe positive Z (+Z) direction from the laser to the disk and completelyin the negative Z (−Z) direction from the disk to the multi-elementphotodetectors. In this manner, three or more transmissive DOE surfacesare used, in which three DOE surfaces are shown in FIG. 11. Thetransmissive DOE surfaces are used to focus the light beam from thelaser diode onto the disk with high NA. The DOE surface 1′ expands thedivergence of the light beam from the laser diode so that the light beamcan achieve its desired size in a shorter optical path length. The DOEsurface 2′ and 3′ share the work of focusing the light beam so that thedesired high-NA converging beam is incident on the disk 26. Thediffractive power of the DOE surfaces 1′, 2′, and 3′ are designedtogether to minimize the physical size of the pickup and to share therequired focusing power such that a high diffraction efficiency,reasonable minimum grooving spacing, and the necessary degree ofachromaticity can be achieved.

As in embodiment 1, the polarization-selective DOE 44 d is oriented suchthat it does not diffract the linearly polarized light travelling fromthe laser diode to the disk 26. The quarter-wave retarder 44 c isoriented such that when the light beam passes through the quarter-waveretarder 44 c twice, once on the way towards the disk 26 and once on thereturn trip after reflection from the disk, the linear polarizationdirection of the laser light is rotated by 90 degrees. This is thepolarization state for which diffraction by the polarization-selectiveDOE 44 d is maximized. The polarization-selective DOE 44 d, as inembodiment 1, serves the role of a beamsplitter, diffracting the lightbeam onto the multi-element photodetectors, and also the role ofservo-generating elements, altering the amplitude and phase of thediffracted light field in such as way that appropriate FES and TESsignals can be extracted from the electrical signal generated by themulti-element photodetectors. Alternatively, as in the previousembodiment, the polarization-selective DOE 44 d and the quarter-waveretarder 44 c can both be replaced by a singlenon-polarization-selective DOE that implements the servo-generating andbeamsplitting functions, albeit with less efficiency than the two-layermethod. Any of the standard servo-generating approaches can be usedincluding, but not limited to, the particular method described above(FIG. 9) or the astigmatic, knife-edge, or spot-size detection methodsfor generating FES, and the pushpull or DPD method for generating theTES can be used in this design. The RF signal containing the informationstored on the disk is the sum of the electrical signals from all of themulti-element photodetectors.

Embodiment 3.

Embodiment 3 is similar to embodiment 1 except the edge-emitting laserdiode 52 and the turning mirror/beamshaper on the semiconductorsubstrate are replaced by a vertical cavity surface emitting laser(VCSEL) source. The VCSEL is a type of semiconductor laser that emitslight from the top flat laser chip surface rather than from its edge.Since it emits light form its top surface, the VSCEL can be mounted flatwithin the well in the semiconductor substrate and the light naturallypropagates towards the diffractive stack. No turning mirror is required.Furthermore, the light beam emitted by the VCSEL has a circularcross-section and no astigmatism. Therefore, the beamshaper is also notnecessary.

Embodiment 4

The invention also provides the embodiment 4, which is similar toembodiment 2 but the edge-emitting laser diode and the turningmirror/beamshaper on the semiconductor substrate are replaced by a VCSELsource.

The invention provides four embodiments above with several advantages. Atraditional style discrete component optical pickup head for disk drivesis shown in FIG. 1 and FIG. 2. Every component, laser, lens,beamsplitter, mirror, etc., are discrete and separately mounted in alarge housing. Another pickup design sometimes known as a holographichybrid pickup is shown as prior art in FIG. 3. This conventional designuses a diffractive or holographic element to serve the dual functions ofbeamsplitter and servo-generating element. However, it still requiresthe separate assembly of many of the components within the holographicmodule and must be mounted in a pickup housing with a separate objectivelens giving an overall size and weight comparable to a traditionaldiscrete component pickup design. The present invention gives a completepickup with the same functionality as the traditional discrete componentdesign or holographic hybrid design, but with a small fraction of thesize and weight. Due to this advantage, the present pickup enables thedesign of disk drives of smaller physical size or the use of multiplepickups within a single drive, for example, to read multiple disksurfaces. Moreover, unlike the traditional or holographic hybridpickups, which require separate assembly of each pickup, the presentinvention is conducive to mass production of many pickups in parallel.The primary optical and optoelectronic elements are all produced usinglithography and standard batch processing procedures, so that manypickups are made at once rather than one at a time. This willsignificantly reduce the cost of producing each pickup.

A prior art pickup design using two mirrored flat surfaces and a singleDOE surface has been disclosed in the U.S. Pat. No. 5,621,716. In U.S.Pat. No. 5,621,716, the optical path length is reduced by folding itusing reflective surfaces. Another prior-art pickup design using twotransmissive DOE surfaces has been disclosed in the U.S. Pat. No.5,805,556. The first DOE surface, nearest to the laser, serves the dualfunctions of expanding the divergence of the laser beam so as to shortenthe optical path length and providing beamsplitter/servo-generatingfunctions for the return beam after reflection from the disk. The secondDOE surface focuses the expanding beam at high NA onto the disk.However, these two prior-art pickup designs above still are notefficient designs. The present invention provides new pickup designswith many important advantages over the prior-art designs.

In conclusion, the present invention includes several advantages asfollows:

1. The invention has at least an advantage of diffractive reflectivesurfaces over planar reflective surfaces. Embodiment 1 of the inventionuses diffractive reflective surfaces to expand the beam divergence whilefolding the optical path. In this way, the optical path length and thesize of the pickup can be made appreciably smaller than the prior-artdesign because the prior art provides no means for expanding the beamdivergence.

2. The invention has at least an advantage of sharing the multipleoptical functions between multiple diffractive surfaces. The prior arthas only a single diffractive surface to provide all of the opticalfunctions. The optical functions include focusing the light beam at highNA onto the disk, and serving as the beamsplitter/servo-generatingelement, transforming the beam returning from the disk such that itsintensity pattern on the signal photodetectors allows the FES, TES andRF information signal to be extracted. There are many advantages to theapproach used in the present design where a stack of multiplediffractive surface shares the multiple optical functions.

3. The invention has at least an advantage of maintaining acomparatively large minimum groove spacing. For a reasonable lens NA,such as NA=0.6 used in digital versatile disk (DVD) system, the minimumgroove spacing required for focusing the light with a single diffractivesurface is less than a micron. The diffraction efficiency is unavoidablyand dramatically reduced when the groove spacing for a lens becomes soclose to the wavelength of light. Thus the effective profile of the beamthat is being focused is distorted due to high efficiency at the centerof the lens where the groove spacing is large and low efficiency at theperiphery of the lens where the groove spacing is small. Ultimately,this produces a poorly focused spot on the disk. With a stack ofdiffractive surfaces, the minimum groove spacing for each surface can bekept large enough to avoid the distortion due to non-uniform diffractionefficiency.

4. The invention has at least an advantage of using a separate DOEsurface for the beamsplitter/servo-generating function. There is also asignificant advantage in using a separate DOE surface different from thediffractive lens surfaces, as in the present invention, to implement thebeamsplitter and servo-generating element functions. Combining all ofthe optical functions into the same diffractive surface as in the priorart both necessitates sharing the diffraction efficiency between thelens and beamsplitter/servo-generating functions and unavoidably createsspurious diffraction orders arising from crosstalk between the twodissimilar optical functions. Even with ideal performance, the bestefficiency that can be achieved would be less than 25%. (This isestimated by assuming 50% of the light into the lens focusing function,50% of the light into the beamsplitter/servo-generating function. Alsoand, only half the light goes into the focused spot on the disk and onlyhalf of that is steered by the beamsplitter onto the photodetectors.) Infact, the power into the spurious diffraction orders due to crosstalkwill also be significant so that the efficiency will be even less thanthe ideal values. Furthermore, the spurious diffraction orders give riseto significant stray light in the system that may degrade the pickupperformance. All of these problems are avoided by using a separatepolarization selective DOE surface for the beamsplitter/servo-generatingfunction. Due to its polarization-selective nature, there is nodiffraction by this surface as light propagates towards the disk. Thusthe diffraction efficiency can be maintained at near 100% for both thelight being focused onto the disk and the return light steered onto thephotodetectors. Furthermore, since the invention implements the lens andthe beamsplitter/servo-generating optical functions using independentDOE surfaces, there will be no crosstalk between these functions, andtherefore no spurious diffraction orders to waste light and createstray-light noise problems.

5. The invention has at least an advantage of using multiple DOEsurfaces to compensate chromatic aberration effects. Diffractivesurfaces are known to be very dispersive. Diffraction is a strongfunction of wavelength. In the operation of a pickup head, wavelengthshifts of the laser due to aging and changing temperature areunavoidable. This may degrade the performance of the diffractiveelements. By using multiple DOE surfaces to implement the diffractivelens as in the invention, positive and negative focusing powers can bebalanced between the multiple surfaces so as to provide the desireddegree of achromaticity. This is impossible if only a single diffractivesurface is used as disclosed in the prior art design.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

What is claimed is:
 1. A method for fabricating a stacked optical pickuphead, the method comprising: providing a substrate, which comprises aplurality of photo-detecting systems arranged in a cell array, and eachof the photo-detecting systems comprises a light source and amulti-element photodetector formed thereon, wherein the light source andthe multi-element photodetector are used in the stacked optical pickuphead to allow accessing data stored in an optical data storage medium ata desired position; performing a fabrication process to form apreliminary diffractive optical element (DOE) stack, which comprise aplurality of functional layers used for performing a desired function ofa DOE stack, wherein the preliminary DOE stack comprises a plurality ofDOE cells arranged in a regular array associating in space with the cellarray of the photo-detecting systems; slicing the array of the DOE cellsinto a plurality of individual strips of DOE cells; precisely locatingthe individual strips of DOE cells on the substrate so that the DOEcells are aligned to the cells of the cell array of the photo-detectingsystems; dicing the substrate and the preliminary DOE stack into aplurality of individual stacked pickup heads with respect to the DOEcells.
 2. The method of claim 1, wherein the functional layers of thepreliminary DOE stack are fabricated by steps comprising: forming atleast one lower diffractive lens layer, which contacts with thesubstrate and provides at least one diffractive DOE surface; forming abeamsplitter/servo-generating DOE layer on the at least one lowerdiffractive lens layer, which optionally may or may not be apolarization-selective DOE layer; forming a quarter-wave retarder layerif the previous beamsplitter/servo-generating DOE layer is apolarization-selective DOE layer to rotate a polarization direction ofpassing light, wherein the polarization-selective DOE layer and thequarter-wave retarder layer are used together to diffract a portion of alight beam reflected from the storage medium onto the multi-elementphotodetector; and forming at least one upper diffractive lens layer onthe quarter-wave retarder layer, used to provide at least onediffractive DOE surface, wherein the diffractive DOE surfaces of thelower and the upper diffractive lens layers are together used to expandlight convergence and to achieve a desired focusing power.
 3. The methodof claim 2, wherein patterns of the DOE cells of the functional layersof the preliminary DOE stack are formed by processes comprisingphotolithography and etching.
 4. The method of claim 1, wherein thesubstrate comprises a semiconductor substrate.
 5. The method of claim 1,wherein the light source on the substrate comprises a laser source. 6.The method of claim 5, wherein the light source comprises an edgeemitting laser diode associating with a turning mirror/beamshaper formedthereon also for turning a light direction from the laser diode to theDOE stack.
 7. The method of claim 5, wherein the light source comprisesa vertical cavity surface emitting laser (VCSEL).